Photoelectrons ejected by normally incident linearly polarized He i radiation and emitted normal to epitaxial Au layers on Pt(111) are strongly spin polarized for both emission from the Pt substrate and the Au adsorbate up to 2.2 layers coverage. Relativistic Green’s-function calculations reproduce the measured data and reveal the physical origin of the polarization for a monolayer Au coverage to be a Au-induced surface state. The spin polarization is significantly influenced by the underlying Pt bulk and is thus a sensitive measure of the Au adsorption site.